1. Field of the Invention
The present invention relates to a process for modifying a surface of a fluororesin product, and more particularly, it relates to a process capable of readily preparing a fluororesin product which is improved in wettability of its surface etc. in a short time by irradiating the surface thereof with synchrotron radiation.
2. Description of the Background Art
Fluororesin has excellent chemical resistance, heat resistance and durability, high electric insulation, and electric characteristics such as a low dielectric constant. A surface of fluororesin is inferior in wettabitity, and has water repellency and oil repellency. Due to such excellent characteristics, fluororesin is widely employed in various industrial fields.
While fluororesin products may be directly employed as moldings having various shapes, it is sometimes desired to apply printing or paint onto a surface of some fluororesin products before they are put into practice. Furthermore, while fluororesin is generally employed as an independent or singular material, it may also be bonded to or laminated with another material, to be employed as a composite material.
Due to the inferior wettability of fluororesin, however, it is difficult to print or paint a surface of a fluororesin product. Disadvantageously, a print or paint film that has been applied on a surface of a fluororesin product can be easily separated from the surface. Further, it is also difficult to bond or laminate fluororesin to or with another material. In a composite material including fluororesin, the fluororesin material is easily separated from the other material whereby the quality of the composite material is deteriorated. In a fluororesin molding such as a container, it is a further disadvantage that bubbles appearing on or adhering to its surface are difficult to remove or separate therefrom.
On the other hand, fluororesin is also molded into porous materials, and applied to various filters. Such a filter of fluororesin may be generally provided with a hydrophilic function on surfaces of its pores, in response to its object and application.
In order to solve the aforementioned problems of the conventional fluororesin products, there have generally been proposed various methods for improving the wettability etc. of the fluororesin surfaces. Among such methods, a method of modifying a surface of a fluororesin product by irradiating the surface with light and causing a photochemical reaction therein is particularly noted, because it causes no problems in its process chemicals.
Japanese Patent Publication No. 3-57143 (1991) discloses a method of irradiating a surface of a polytetrafluoroethylene product with a pulsed beam emitted from an excimer laser having a wavelength region of 95 to 200 nm. On the other hand, Japanese Patent Laying-Open No. 3-259932 (1991) discloses a method of irradiating a surface of a fluororesin product with vacuum ultraviolet light having a wavelength of not more than 190 nm. In this method, the vacuum ultraviolet light is emitted from a rare gas discharge plasma light source. Further, Japanese Patent Laying-Open No. 3-269024 (1991) discloses a method of irradiating a fluororesin product with ultraviolet light having a wavelength of not more than 1800 .ANG..
On the other hand, "Genshiryoku Kogyo" Vol. 39, No. 4 (1993), pp. 56 to 59 discloses a process of previously adding a substance, such as total aromatic polyester or polyether ketone, that absorbs light having a wavelength of at least 200 nm to a fluororesin product and thereafter irradiating a surface of the fluororesin product containing the light absorptive substance with light having a wavelength of 248 nm emitted from a KrF excimer laser beam source. The KrF excimer laser is a Light source that can obtain a high output with stable oscillation. According to this method, it is possible to treat the target in the atmosphere at ordinary temperature, without requiring a large reaction vessel and a vacuum unit. Therefore, this method is regarded as being suitable for industrialization. In this method, however, it is impossible to effectively modify a product of resin that consists of only a C-X bond (X: hydrogen, nitrogen or fluorine) containing no light absorptive substance.
A surface of a fluororesin product which is modified by a conventional method is disadvantageously insufficient in hydrophilicity and adhesiveness. FIGS. 4(a) and 4(b) are sectional views typically showing surfaces of resin products prepared by conventional methods. Referring to FIG. 4(a), numeral 110 denotes a fluororesin molding, numeral 110a (in a two-dot chain line) denotes an unmodified surface, numeral 110b denotes a modified surface, numeral 111 denotes a modified layer, symbol d denotes the thickness of the modified layer 111, and arrow 50 denotes light which is applied for the modification. In the conventional method, the light 50 merely slightly penetrates into the interior of the fluororesin molding 110, and hence the modified layer 111 has a small thickness d. The thickness d of the modified layer 111 that can be formed by the conventional method is about 1 .mu.m at the most. A texture formed on a surface of a fluororesin product by a conventional method is so small that a printing ink, a paint or another material can hardly penetrate into the surface. Thus, such a material insufficiently adheres to the fluororesin product.
FIG. 4(b) typically shows a section of a porous fluororesin molding whose surface is modified by a conventional method. Referring to FIG. 4(b), numeral 120 denotes the porous fluororesin molding, numeral 120a (in a two-dot chain line) denotes an unmodified surface, numeral 120b denotes a modified surface, numeral 122 denotes pores of the molding 120, numeral 122R denotes surfaces of the pores 122, and arrow 50 denotes light which is applied for modification. Using this conventional method, the light 50 merely reaches a portion which is close to the surface 120b as irradiated, and hence the pores 122 are insufficiently modified. Therefore, a technique of modifying the overall pores of a porous material such as a filter, has generally been awaited in the field.